The rails move easily by themselves, and the focusing gear isn't binding.
But when I cleaned and lubed them and put them back together, there's no improvement. The front standard is timed correctly, too.

I assume that the pinion gear starts to slip on the track and not advance the front standard.

When you had it apart was there any noticeable wear on the pinion or the tracks? Measure the track and pinion with a caliper.

Is there any play in the pinion? I seem to remember a post about adding a washer between the focus knob and body guide to tighten a loose focusing knob or to prevent the focus from drifting when the camera was tilted. Use a brass washer.

Check the squareness of the focus track to the camera body with a square and the front standard at several distances from the body.

If the focusing is slipping at the center and good either side that is an indicator that the track is worn down. If the slippage starts around center and continues to full extension then the pinion is not engaging the track due to mis alignment of the track to the body.
Are the focus tracks removable from the rails that they are on? If so and squareness of the rail is not at fault try putting a .001 to .002 inch thick shim between the track and rail in the section that slips._________________The best camera ever made is the one that YOU enjoy using and produces the image quality that satifies YOU.